The present invention relates to the manufacture and use of solid, intumescent, phosphorus-containing flame retardants.
Intumescent agents have a flame-retarding effect in that under more intense heat, for example, under the effect of a fire, they expand into a heat-insulating, low-flammability coke layer. Thus, they suppress, inter alia, the dripping of molten, possibly burning material.
Intumescent flame-retarding systems are known, especially for painting compositions and coatings. In certain individual cases, such systems are also utilized in thermoplastic polymers. In general, the intumescent mixture consists of the following components:
(a) a salt of phosphoric or polyphosphoric acid (for example, ammonium phosphate), PA0 (b) a polyhydroxy compound (e.g., pentaerythritol), PA0 (c) a nitrogen compound (e.g., melamine), and PA0 (d) optionally, a halogen compound (e.g., chloroparaffin).
(See, e.g., D. G. Brady, J. Fire Retard. Chem., 4 [1977] 3: 150.)
Intumescent coating compounds based on ammonium polyphosphate are disclosed, for example, in DOS's [German Unexamined Laid-Open Applications] Nos. 1,794,343 and 2,359,699. Because of their use of an inorganic polyphosphate, however, such compositions exhibit disadvantages when mixed with thermoplastics and lead to poor physical properties of the resultant synthetic resin. The flame-retardant mixtures for polypropylene proposed in DOS No. 2,800,891 likewise contain ammonium polyphosphate or aminophosphates and, thus, are burdened by the same deficiencies.
DOS No. 2,506,096 describes the treatment of cellulosic material with the reaction products of aqueous phosphoric acid and ethylene oxide. These are neutralized and applied to the cellulose in the dissolved condition. Such solutions, or also concentrated oily or viscous masses made therefrom, are unsuitable for thermoplastic processing. In particular, their water solubility constitutes a special disadvantage since the agent, in the presence of water, is removed to the greatest degree from the marginal layers wherein it is especially to be effective.
In contrast thereto, U.S. Pat. No. 4,010,137 describes flame-retardant compositions, for example, of polypropylene, which contain a mixture, prepared at an elevated temperature, of polyphosphoric acid, pentaerythritol and melamine. However, this flame retardant must be produced under considerable mechanical agitating power in the melt at temperatures of up to 250.degree. C. A further disadvantage is that the product must additionally be thermally degassed to be able to be incorporated into the synthetic resins using the customary processing temperatures without danger of foaming. In this treatment, the agent not only loses a substantial part of its actually desirable foamability, but also loses its white color and becomes yellowish to brown.
Therefore, there is still a need for an intumescent flame-retardant formulation for thermoplastic synthetic resins, such as polypropylene, distinguished by an easy method of manufacture, and by possession of the properties of being colorless, conveniently incorporable into polymeric compositions, compatible, and highly efficacious as a flame retardant, all at the same time.